1. Field of the Invention
The present invention relates to a cathode carbon block for an aluminum electrolytic cell prepared by baking raw materials mainly comprising petroleum coke at a high temperature and more particularly, the invention relates to a cathode carbon block for an aluminum electrolytic cell having a stability constant of 0.7-1.0 and capable of continuing electrolysis over a long period of time.
2. Description of Prior Art
A carbon cathode for an aluminum electrolytic cell is constructed by lining carbon blocks baked preliminarily on the bottom of an electrolytic cell. As is well known, the electrolytic production of aluminum is generally carried out by dissolving alumina in a molten salt bath mainly composed of cryolite at about 940.degree. C. in an electrolytic cell and passing a DC current through the electrolytic cell.
Because the carbon cathode is subjected to such severe conditions such as the passage of electric current, the high temperature and contact with a molten salt and molten aluminum at the bottom of the cell, the carbon cathode is gradually deteriorated. The deterioration of the carbon cathode is also caused by the penetration of the molten salt bath as well as by the violent attack of the initial current passing through and thermal shock. The deterioration of the carbon cathode is accompanied by certain defects which prove unprofitable in operation and economy, such as the increase in electric resistance, the unevenness in electric current distribution, and the reduction in current efficiency.
A carbon block for a cathode of an aluminum electrolytic cell is usually produced by calcining a raw material mainly comprising anthracite at about 1300.degree. C. Recently, the use of a carbon cathode having graphite compounded therein has been attempted. Although the swelling property of the cathode by the penetration of sodium in a molten salt bath may be improved by the addition of graphite, the remarkable effect of greatly prolonging the life of the cell cannot be obtained by using such cathode carbon block.
Although the characteristics of the cathode carbon block give decisive influences on the life of the electrolytic cell, the characteristics of the cathode desired are ambiguous and the main factors for producing the carbon cathode satisfying the aforesaid purposes are also obscure. Illustrative of such characteristics desired for the carbon cathode for an aluminum electrolytic cell are a fine structure, less microporosity, a high spalling resistance, favorable mechanical properties, a low specific resistance, and a high chemical stability. In general, it has been determined that the carbon cathode has an apparent specific gravity of higher than 1.55, a true specific gravity of higher than 1.90, a porosity of less than 20%, a specific resistance of less than 500 .times. 10.sup.-.sup.5 ohm-cm, a compressive stength of higher than 300 kg/cm.sup.2, and a tensile strength of higher than 20 kg/cm.sup.2. However, even in the case of using a carbon cathode satisfying these standard factors, the life of the aluminum electrolytic cell can be increased to only about four years on the average.